1. Field of the Invention
The invention relates generally to electrodeless fluorescent lamps and more particularly to an improved shape for the envelope of an electrodeless fluorescent lamp.
2. Description of Related Art
In the prior art electrodeless fluorescent, or low pressure gas discharge, lamp illustrated generally in FIG. 1, an induction coil inserted into the lamp cavity drives a magnetically induced plasma discharge which circulates around the coil around the axis of the lamp in the direction of the electric field created by the oscillating current and essentially in the direction of the coil windings.
Because the coil is necessarily not very large, e.g. less than 2 cm in diameter, and because the discharge is close to the coil for good coupling, the effective arc length is small, on the order of less than 10 cm compared with about 60 cm for typical 20-25 watt compact fluorescent lamps (CFL) with electrodes. Hence, to have a 20-25 watt lamp, the arc current must be very high, about 3.5-4.0 A compared to 0.16-0.32 A for typical CFLs. A large arc cross sectional area must be allowed since if the current is constricted by the walls of the discharge then there will be a high current density with a significant decrease in efficiency due to wall losses. Also, there will be a further loss in light output as a function of time due to a high ion bombardment rate on the wall coating.
The prior art electrodeless fluorescent reflector lamp illustrated generally by FIG. 1 has the shape of a standard, R80, blown bulb incandescent reflector lamp. The top is a semi-prolate ellipsoid with the major axis being about 80 mm. The flattened ellipsoid is connected with a radius of curvature to a paraboloid reflector shape forming the base or reflector face of the lamp. This paraboloid reflector region is designed to focus light from an incandescent filament into the forward direction. However, when used as the reflector region for an electrodeless fluorescent lamp, it is covered first with a diffuse reflector and then a phosphor powder coating. A capped cylindrical glass tube (52 mm.times.26.5 mm) is inserted into the base of the reflector bulb and sealed to it. This covers the ferrite induction coil which drives the plasma discharge. See FIG. 1.
Since the bulb is fairly large and much larger than the inserted cylindrical glass tube, the prior art design does allow a relatively large cross section for the plasma discharge. However, being designed as an incandescent reflector lamp it is not optimized for maximum efficiency for an electrodeless discharge lamp.
There is a need for a shorter lamp with an improved shape. A shorter lamp will allow the maximum overall height of the lamp plus ballast to be the same as for a standard incandescent R80 bulb, insuring that it does not protrude out of fixtures made for the latter. Reducing the height of the bulb will allow more room for electronic ballast components where space is at a premium. Furthermore, by reducing the height of the lamp one also reduces the amount of glass and phosphor materials required to make the lamp.
Simply scaling down the height of the prior art lamp, however, would constrict the discharge and reduce its efficiency significantly. There is a need for a new shape for the bulb, which allows the height of the bulb to be reduced without significantly reducing efficiency or increasing wall loading.